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For the past few months our flying club has been plagued with a problem. The problem is that when we start the plane, we initially have a good surcharge on the ammeter. Then, either we run-up, takeoff, or in-flight the ammeter all the sudden displays a 5 amp discharge.

Turning the master switch off and on seems to temporarily fix it, but it comes back in minutes. The alternator, regulator, battery, self-resetting field circuit breaker, and over-voltage sensors have all been replaced within the last few months in attempt to fix the problem. But still this problem persists.

Has anyone had any similar issues or have any guess what could cause this?

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    $\begingroup$ Sounds like maybe a loose connection/intermittent short. $\endgroup$
    – Jim
    Commented Jul 22, 2022 at 2:22
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    $\begingroup$ Can you prove it is truly discharging the battery during flight? It almost sounds like the ammeter (shunt) is reversed. $\endgroup$
    – 300D7309EF17
    Commented Jul 22, 2022 at 2:55
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    $\begingroup$ Track down the consumer. $\endgroup$
    – Koyovis
    Commented Jul 22, 2022 at 4:59
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    $\begingroup$ Is the time between master on and the 5 A drain appearing constant? If so it might give a clue to what's pulling the current. What kind of avionics do you have? A competent tech (I am not) should be able to solve this... $\endgroup$
    – TypeIA
    Commented Jul 22, 2022 at 5:31

3 Answers 3

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The generator on the engine can put out lots more than 5 amps, so if a 5 amp discharge load suddenly presents itself and the rest of the charging/battery system is functioning normally then that load will be picked up by the system and no discharge will show on the ammeter.

This means that the charging circuit has stopped operating, and the 5 amp load could just be representing the normal lighting/avionics house load, which then runs the battery down. (Next time the 5 amp load pops up, try pulling the breakers one by one until you find which load branch is producing it.)

Proof that the charging system is faulty can be gotten by putting a voltmeter across the battery terminals. Any time the generator system kicks on, it will show about 14.5-14.8 volts and positive current ("+ charge") on the ammeter and anytime it cycles off the ammeter will show negative current ("- discharge") and somewhere around 11.8 volts.

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  • $\begingroup$ All good stuff. The system is actually a 28v system. We believe the problem is RPM dependent seems like the alternator trips when the rpm is >1700 for some reason. Master switch reset seems to temporarily fix it. Also, no high-voltage light. $\endgroup$
    – Preston
    Commented Jul 22, 2022 at 13:50
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The last thing to check is wiring. It sounds like an intermittent short to ground that passes through a high resistance load path of some kind first when it makes contact, such that it acts like you turned a heater element on or something, and doesn't trip the breaker.

The wiring harness needs to be "wrung out", using a Digital Multimeter, first for shorts that appear spontaneously when the wire harnesses are jiggled at various places in the charging circuit.

If that doesn't find anything, the next test is Megger, or Meg-Ohmmeter checks, which puts high voltage/low current through wiring runs disconnected from the services, looking for MegOhm values that point to current leakage through the wiring insulation.

Usually you will put 200 or maybe 400 v though the disconnected wires, via the multimeter (milliamps of current), with resistance below some threshold indicating insulation breakdown.

Megger is a very common troubleshooting procedure on airliners, but may be rarely done on light airplanes, so I don't know if you'll even be able to find Megger specifications for the airplane's wiring harness in its documentation.

On transport airplanes, the normal sequence is throw parts at it, problem still comes back, wring out the wiring looking for direct shorts, then insulation breakdown checks as I described.

A 45 year old airplane is likely to have brittle wiring insulation, especially on the engine side of the firewall where all the heat is.

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  • $\begingroup$ super-knowledgeable answers here $\endgroup$
    – Fattie
    Commented Jul 22, 2022 at 17:38
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I have the same problem with a 152 model 1978. There is a small relay hidden behind the trim console by the firewall. It is small black box about 2" square.

That is your problem.

The same is with a 172 M model that i had but the tiny relay was in the wiring by the master switch.

good luck

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